The present invention relates to implantable cardioverters/defibrillators generally, and more particularly to an implantable bi-atrial pacemaker/defibrillator with capability of atrial fibrillation reoccurrence prevention.
Early concepts of implantable defibrillators, such as those disclosed in Reissue Pat. No. 27,652 by Mirowski, et al, envision an electrode system employing a ventricular endocardial electrode and a plate electrode mounted to the heart directly, subcutaneously, or to the skin. However, it has long been recognized that a totally transvenous system would be desirable in order to simply the use of implantable defibrillators. One such system is suggested in Mirowski, et al U.S. Pat. No. 3,942,536, which discloses a transvenous lead having electrodes intended for location in the right ventricular apex and in the superior vena cava. This electrode system is disclosed as useful for either ventricular or atrial defibrillation.
While systems employing epicardial patch electrodes are workable, a thoracotomy is required in order to apply the epicardial electrodes. It is generally believed that it would be desirable to produce an implantable defibrillation system that entirely avoids the necessity of a thoracotomy. To this end, there has been substantial work directed towards development of such systems, as disclosed in Kallok U.S. Pat. No. 4,727,877, Tacker, et al, U.S. Pat. No. 4,708,145, and U.S. Pat. No. 5,014,696 by Mehra, for an “Endocardial Defibrillation Electrode System”. Other types of endocardial defibrillation electrodes are disclosed in Gold et al U.S. Pat. No. 4,481,953, Kinney, et al U.S. Pat. No. 4,161,952, Kiekhafer et al U.S. Pat. No. 4,934,049, and in U.S. Pat. No. 5,042,143 by Holleman, et al, for an “Method for Fabrication of Implantable Electrode”. The Kinney, Gold, Kiekhafer, and Holleman patents all disclose endocardial defibrillation leads employing defibrillation electrodes fabricated from elongated coils of biocompatible metal, mounted exposed to the exterior of the defibrillation lead, for placement in the right ventricle and other locations within the heart. The above-cited Mehra patent discloses a variety of endocardial defibrillation electrodes intended for use in the atrium, ventricle and coronary sinus, all of which employ electrodes in the form of elongated coils of conductive biocompatible metals.
Concurrent with the development of lead systems adapted to treat ventricular fibrillation, there has also been some work directed to the development of lead systems to treat atrial fibrillation. Synchronized cardioversion using two electrodes located on a lead located in the right atrium is disclosed in Charms U.S. Pat. No. 3,738,370. A later system is disclosed in Mirowski et al U.S. Pat. No. 3,952,750, employing one electrode in the atrium and presumably a second electrode at an unspecified location.
An electrode lead system specifically designed for atrial defibrillation is disclosed in the article “Elective Countershock in atrial Fibrillation With an Intracardiac Electrode—A Preliminary Report, by Jain, et al, published in the Journal of the Association of Physicians of India, Vol. 18, pp 821–824, 1970. The lead in the system was provided with a 10 mm silver electrode for location in the right atrium and was tested in conjunction with either a second electrode located in the right atrium or a second, cutaneous electrode located on the left side of the chest wall. A second electrode system specifically designed for use in atrial cardioversion is disclosed in the article “Safety and feasibility of transvenous cardioversion in atrial tachycardia”, by Blanc et al, published in Cardiac Pacing, edited by Gomez, Futura Pub. Co., 1985, pp 1526–1529. This electrode system employed a single lead with electrodes located in the atrium and pulmonary artery.
Atrial defibrillators are also known in the art. Specifically U.S. Pat. No. 5,165,403 “Defibrillation lead system and method of use” to Mehra describes a method of cardioverting the atrium of a human heart that includes insertion of first and second elongated electrodes tranvenously into the heart and associated vessels. One electrode is preferably located in the coronary sinus and great vein of the heart. The other electrode is preferably located in the vicinity of the atrium of the heart, spaced from the electrode located in the coronary sinus. In response to detection of fibrillation or in response to manual triggering, a defibrillation pulse is applied between the first and second electrodes to effect atrial cardioversion. Additionally, U.S. Pat. No. 5,433,729 “Atrial defibrillator, lead systems, and method” to Adams describes an implantable atrial defibrillator providing a pulse of defibrillating electrical energy to the atria of the heart in synchronism with sensed R waves in response to non-coincident sensing of an R wave at first and second areas of the heart. The defibrillating pulse is provided after a predetermined number of consecutive R waves are non-coincidently sensed to assure reliable synchronization. The atrial fibrillation detector of the defibrillator is normally disabled and is activated when the sensed ventricular activity indicates a probability of atrial fibrillation to conserve a depletable power source. A plurality of lead systems are also described for use with the atrial defibrillator which reduce the quantity of electrical energy required to defibrillate the heart and ensure that the delivered atrial defibrillating electrical energy is substantially confined to the atria of the heart. The '403 and '729 are incorporated by reference herein in their entireties.
In many patients, upon successful termination of atrial fibrillation by cardioversion and/or defibrillation, the immediate or near immediate reoccurrence of atrial fibrillation is a known problem. For example, Tse, et. al. describes a difference in p-wave width after low energy shocks in some patients in “Reoccurrence of Atrial Fibrillation after Successful Cardioversion with Transvenous Atrial Defibrillation is Associated with Prolonged P Wave”, European Journal of Cardiology, Pacing and Electrophysiology, Vol 6, No 1, Page 55, June 1996. The authors' hypothesis is that some patients have a propensity to have a temporary increase in dispersion of Intra-Atrial Conduction Delay (IACD) that is an indicator of re-initiation of atrial fibrillation.
Standard defibrillators as described above (Mehra '403 and Adams '729) would re-detect the re-initiated fibrillation and re-shock the patient. If multiple re-initiations occurred, this would cause multiple fibrillation detections and treatments causing shortened defibrillator life, patient discomfort, and reduced quality of life. What is needed is a defibrillator that initiates preventive therapy after a successful cardioversion/defibrillation shock to prevent reoccurrence or re-initiation of fibrillation upon successful termination of an arrhythmic episode.